The present invention is an improved construction for a spray gun, and is particularly adaptable for the construction of a paint spray gun operated under both the influence of pressurized air and pressurized spray liquid. More particularly, the invention is directed toward an improved nozzle design for utilization with a conventional airless spray tip of the type having an elliptical orifice through which liquid is sprayed under the influence of high liquid pressures.
In the field of liquid spraying, and more particularly in the field of paint spraying, it is desirable to produce a fan-shaped spray pattern of liquid particles which are finely and uniformly atomized. It has long been recognized as desirable that the spray pattern emanating from such a spray gun be elongated and generally elliptical in shape, rather than circular or some other shape. The elliptical, fan-shaped pattern enables a skilled operator to apply uniform paint coatings over both large and small surfaces.
In the art of spray painting, it is typical to spray at a distance of 6-18 inches from the article to be coated, and conventional airless spray tips typically develop a spray pattern width at the article to be coated in the range of 2-18 inches. In developing such a pattern, it has been long recognized that the critical problem area with respect to the development of a uniform pattern has occurred at the extreme edges of the fan-shaped pattern. For reasons not completely understood by those skilled in the art, an excess accumulation of paint spray particles tends to travel along either extreme edge of the spray pattern, and if particular care is not taken in the set-up and adjustment of the spray equipment these edge accumulations tend to develop "tails". The development of "tails" on an article being sprayed results in an excess accumulation of paint coating both above and below the main spray pattern, which excess coating is susceptible of collecting into droplets and destroying the uniformity of the coating finish. In an application utilizing airless spray equipment it is usually possible to eliminate such "tails" by merely increasing the liquid paint pressure in the system. However, this results in other adverse effects, for in typical operating equipment it is usually necessary to operate at liquid pressures in the range of 1500-3000 pounds per square inch (p.s.i.), requiring the design of equipment capable of withstanding such pressures.
Efforts directed toward solving the problem of providing uniform spray under lower pressure airless paint spraying conditions resulted in the first successful invention accomplishing this, are disclosed in U.S. Pat. No. 3,843,052. In this patent, it was revealed that a low-pressure air stream could be directed at the flat liquid fan emanating from the orifice at a point prior to the zone of atomization of the fan, and this would enable a significant reduction in the liquid pressures required for uniform atomization. Subsequently, other patents issued which disclose various forms of air jet construction for accomplishing this end. For example, U.S. Pat. No. 3,907,202, issued Sept. 23, 1975, discloses an airless spray gun having an annular air orifice concentric to the liquid paint orifice, and at least two air orifices disposed on either side of the nozzle for directing air jets onto the flat fan-shaped paint film emanating from the paint orifice. U.S. Pat. No. 4,055,300, issued Oct. 25, 1977, discloses a hydrostatic atomizing nozzle having a pair of air orifices disposed on either side of the nozzle to deliver air jets in a plane perpendicular to that of the flat paint film, but directed toward the front surface of the frusto-conical liquid nozzle so as to bounce the air stream off the nozzle prior to contacting the flat paint film. U.S. Pat. No. 4,219,157, issued Aug. 26, 1980, discloses a hydrostatic atomization nozzle having at least two complementary jets of compressed air coplanar with the paint liquid fan and converged towards the paint liquid fan at a point displaced forward of the paint nozzle. U.S. Pat. No. 4,232,824, issued Nov. 11, 1980, discloses a hydrostatic atomizing nozzle having a plurality of air jets directed both at the liquid nozzle surface and also parallel to the liquid nozzle surface, the claimed result of all of the air passages being an improved atomization of the paint pattern.